GoSlash27

Gotta keep in mind that minimal DV expenditure isn't profitable. You can have a stage that expends more DV but weighs less overall by reducing the t/w ratio. You can go even further and make a cheaper stage at the expense of DV *and* fuel consumption. Minimal mass and minimal cost are worthwhile goals when designing stages. Minimal DV expenditure isn't. Best, -Slashy

Actually, that's not true. I design every stage I build this way. Scope out my tutorial here: http://forum.kerbalspaceprogram.com/threads/136367-How-to-mathematically-design-stages for how the math works. Best, -Slashy

Neither. On the first stage the pressing concern is either mass or cost. If you're working in an early career, you may be dealing with a pad mass restriction where the cheapest solution is too heavy to launch. If not, you want to do the mission as cheaply as possible to save money for more lucrative missions down the road. T/W and DV are not competing criteria. You must achieve absolute minimums on both, and having an excess on one will not overcome a deficiency of the other. I design my booster stages to achieve both 1.4 t/w *and* 1,800 m/sec DV. Best, -Slashy

The Skipper is an excellent first stage for early career. If your upper stage and payload is between 6 and 16 tonnes, there's nothing lighter and only SRBs are cheaper... if you've unlocked the good ones. For payloads 18- 45 tonnes later in the game, it's unbeatable as an upper stage to orbit. A very good all- around workhorse engine. I probably use it more often than any other single LF&O engine. Best, -Slashy

Ohio Bob, If your numbers line up, then that's *definitely* confirmation. Yes, I used 60km for Duna and 100 km for Eve. This confirms that the model is working properly. Thanks, -Slashy

ajburges, At supersonic speeds, surface and tail drag are miniscule. The size1->size2 adapter is a very clean shape (.425 Cd) and it occludes the engine itself. The drag penalty is marginal in that case. Adding parallel stacks for the Terrier can also be done cleanly, and when using TJ hybrid designs you don't have much choice in the matter anyway; you have to add nodes. I like to use 2 Terriers for 1 TJ or 1 Poodle for 2 TJs. I agree. In fact, I consider interplanetary spaceplanes to be a self- conflicted notion. The entire point of an SSTO spaceplane is efficiency. There aren't many concepts less efficient than lugging an entire airplane to another planet. However, spaceplanes do need some additional orbital DV to be useful as delivery vans, which is their prime function. They need to be able to correct inclination a couple degrees, intercept and rendezvous with a station in LKO, and deorbit. I design my ships to have at least 250m/sec on orbit. My TJ hybrid designs are pretty darn weak by that measure. I'm down around .5 at takeoff and I've gone as low as .34. Best, -Slashy

During the early rocket powered part of the ascent though, i'd thought it's still better to have a larger wing at low AoA than a small one at high AoA because you'll be thrusting closer to prograde.

FancyMouse, Thanks, I had just noticed that myself Best, -Slashy

MKendall, Yeah, that's what I did. Perhaps I need to also work out a worst case DV to account for eccentricity and inclination... A precise time- based model wouldn't really work for what I'm trying to do. *edit* it looks like my prediction of transfer window intervals is completely wrong. The table you linked has much shorter intervals. *'nuther edit* these tables are in Earth days, not Kerbin days. Looks like it's okay after all... Thanks, -Slashy

Gaarst and FancyMouse, Thanks and I appreciate that, but I'm just looking for confirmation of the model or (more importantly) glaring inaccuracies. Especially the phase angles and durations. The idea is that this model is supposed to be used for mission planning for ship design purposes. I need to make sure it's acceptable so ships designed with it will perform as expected. Either that, or I need to know that it's incorrect so I'm not designing stages that can't perform in the actual mission. Thanks, -Slashy

It used to be that you had a limited amount of time until the Kerbal version of the "Y2K bug" hit, so it was in your best interest to hoover up as much science as possible in limited time. I don't know if that's still the case, but I still operate that way. Best, -Slashy

ajburges, No, not particularly. The Poodle is just plain all- around a better option in installations where the Swivel would be considered. And often people will opt for higher t/w for designs that don't really warrant it because it's easier. Even in those cases, the Terrier can usually still out- perform the Swivel in the same application. It's fine if you just want to make a working spaceplane and don't care about payload fraction, but as far as that goes you can use pretty much anything for an anaerobic stage in a spaceplane. The folks who are building efficient and economical spaceplanes don't use swivels and there's a good reason for that. Best, -Slashy

The scene where he unbolts the airlock from the MAV, muscles it over the side, and just lets it tumble down the side of the stack *cringe*. An Air Force Mechanic once dropped a socket down into a silo containing a Titan II (also hydrazine fueled). It didn't end well. Still, It's just a movie and it was very entertaining. Best, -Slashy

RIC, Ahh, but this is why it's so important to not pass Ap in a low t/w spaceplane You have to be careful about how much you pop up at the transition so you don't wind up in that predicament. I've played around with some ridiculously underpowered spaceplanes (as low as .34 t/w on rockets) and gotten them to orbit. This does incur a penalty to drag losses, though, and I like to keep it at at least .5. Best, -Slashy - - - Updated - - - kStrout, Glad to hear you made it and yeah, that looks about like my standard profile. Yeah, it's easier to circularize with more t/w, but that's a by- product of using more engine than you need rather than a property of the Swivel itself. As you point out, it needs more fuel and oxidizer to do the same job. This is why the spaceplane gurus don't use them; they're all about getting the payload to orbit as cheaply as possible. Best, -Slashy

*SMH* What is with this sudden fascination with the LV-T45?? Scott Manley uses one and all of a sudden people think it's a great engine... The Swivel is an *okay* engine for this job. It is not ideal, though. The best engine is whatever will do the job with the least combined mass of engine, oxidizer, and fuel. The Terrier and Poodle are both much better than the Swivel for this job. Sorry, not venting at you personally. I just keep seeing this everywhere lately and it's bad info. Best, -Slashy

I'm working on a spreadsheet to plan interplanetary missions and need to verify that the numbers I'm getting are correct. Could you folks please look this over and make sure it's close to correct? Kerbin to Duna: A Hohmann transfer window opens once every 909 days, 3 hrs. Duna should be 44° prograde Kerbin at injection burn. Injection burn from LKO is 1,078 m/sec Elapsed time of Hohmann transfer is 302 days. Kerbin should be 75° prograde Duna upon arrival Circularization burn at LDO is 616 m/sec (if I don't aerobrake). Kerbin should be 75° retrograde Duna at departure. Duration of stay to Hohmann transfer window home is 529 days 4 hrs Injection burn home is 616 m/sec from LDO. Elapsed time of Hohmann transfer home is another 302 days. Retroburn at LKO is 1,078 m/sec (if I don't aerobrake). Total mission duration 1,133 days, 4 hrs. Kerbin to Eve: A Hohmann transfer window opens once every 680 days. Eve should be 54° retrograde Kerbin at injection burn. Injection burn from LKO is 1,043 m/sec Elapsed time of Hohmann transfer is 170 days 2 hrs. Kerbin should be 36° retrograde Eve upon arrival Circularization burn at LEO is 1,402 m/sec (if I don't aerobrake). Kerbin should be 36° prograde Eve at departure. Duration of stay to Hohmann transfer window home is 543 days 4 hours. Injection burn home is 1,402 m/sec from LEO. Elapsed time of Hohmann transfer home is another 170 days 2 hrs. Retroburn at LKO is 1,043 m/sec (if I don't aerobrake). Total mission duration 714 days, 1 hour. Thanks, -Slashy

Well... they can become *somewhat* more efficient if done properly, but it's not a huge difference. The advantage of using a single fuel type and high Isp is almost completely countered by the LV-N's 3 tonne mass. For turbojet/ rocket hybrids, I like to stick with the Terrier or Poodle. For RAPIER designs, I don't bother adding specialized rockets. Best, -Slashy

Alshain, If there is even one person here who is making low TWR spaceplanes, then they are by definition not "broken", but rather simply more difficult than they were before. And of course, there are several of us who are doing this. Not only doing it, but doing it at mass efficiencies that conventional rockets can't hope to touch. If we can do it, then so can you. Instead of complaining that they're "broken", I suggest upping your game Best, -Slashy

kstrout, You really want to maintain a shallower climb once you hit 18km altitude or so. The plane will climb on it's own so long as you have sufficient speed. You also want adequate wing lift so that you're not trying to fly the whole spaceplane at high AoA. That generates a lot more drag than lift and also causes high cosine losses. Basically... don't try to throw it into orbit with thrust, fly it into orbit with speed. Best, -Slashy

Robert.G, Landing on the Mun with RCS is easy. We did that as a matter of course back in the day. Nearly every Mun lander was RCS back in 0.18. Launching from Kerbin in KSP 1.04, OTOH, is very difficult. The atmospheric Isp is awful and the thrust requirement is much higher. I recognize (and RIC proves) that it is possible, but I'm not going to build a 600+ part monstrosity just to do it. Best, -Slashy

SaturnianBlue, Have you examined how the main tank drains and where you're feeding the engines from? You want it set up so that the CoM shifts towards the SSMEs in a straight line along their center of thrust as the main tank drains. Best, -Slashy

Sovek, It doesn't matter one way or the other. The Mun's rotation is so slow that the difference is negligible. Best, -Slashy

In my shuttle, I balanced the center of lift much closer to the empty CoM. I also have body flaps at the bottom of the rear of the fuselage to aid in pitch control. Best, -Slashy

This looks like it'd be an excellent basis for a challenge. *evil grin* Mach 1.94 at 10 km with tech level 5 parts. Mach 2.2 at low level It's really pretty simple: The engine can only make use of a certain intake area. It's about .0068 m^2 for the turbojet and RAPIER and about .0017 m^2 for the Wheezley. You want to feed it enough air to keep it from flaming out prematurely. Any more than that is simply adding drag. The object is to feed the engine(s) enough air with as little drag as possible. Most of the intakes in this game are more than enough to feed an engine. I did this using a single engine nacelle for the intake. Best, -Slashy

Case #3: The high thrust LF&O booster stage The priorities get shifted when dealing with boosters. Light weight is no longer a priority since the weight can't cascade down the stack any further. Cost, however, is a big priority because this stage will be destroyed in a stock career launch. Moreover, we are operating in atmosphere now. This adds a couple additional steps to our calculation. We need to figure the minimum acceleration in the worst- case scenario, which is sitting on the pad at sea- level. We also need to assume an average Isp at 50% atmosphere to get a good prediction of our stage size. Aside from that, it's the same process we've used for vacuum stages. Procedure: 1) How much thrust does this engine produce at sea level? 2) What is it's effective Isp over it's flight? 3) How much total mass can a single engine move at my required minimum G? 4) How much of this mass would need to be fuel in order to hit my DV goal? 5) How much payload would this theoretical rocket be able to handle? 6) How many engines do I actually need to handle my payload requirement? 7) How much fuel and tankage? 8) What does it cost? For this example, we will assume a 50t payload. Minimum acceleration is 1.4G and required DV is 1,800 m/sec. We will look at the Twin- Boar in this example. This happens to be the ideal LF&O engine for this job in terms of cost, although the "Kickback" SRB could do it cheaper. Step 1: Thrust at sea level is T(vac)* Isp(atm)/Isp(vac) 2,000 kN* 280s/300s= 1,867 kN The Twin Boar produces 1,857 kN thrust at sea level. Step 2: Average Isp over the duration of the flight is (Isp(atm)+Isp(vac))/2 280s+300s/2= 290s The Twin Boar will average 290s Isp over the duration of it's flight. From here on, it's the same procedure as before. Step 3 The Twin Boar generates 1,857 kN of thrust at sea level. at 1.4g minimum acceleration, this works out to 1,857/(9.81*1.4)= 136 tonnes that a single engine can handle. Step 4 using the reverse rocket equation for 1800 m/sec DV and the Twin Boar's average Isp of 290 sec, 1800/(9.81*290)= 0.633 e^(0.633) = 1.883 Wet to dry ratio To convert this to fuel fraction, it's (1.883-1)/1.883= .469. 46.9% of this example rocket would be fuel. Step 5 Tank mass in this case would be .469/8=.059. our fuel tanks comprise 5.9% of our total stage mass. Adding the fuel and tanks yields .528; 52.8% of our total mass is fuel and tanks. Multiplying this by our capacity yields (136)*.528=71.8 tonnes of tank and fuel and of course our engine is another 6.5t of mass (since the Twin Boar has a free jumbo 64 attached, I subtract the mass and cost of that from the engine) So a single engine's payload capacity would be 136t (lifting capacity) - 71.8t (mass of fuel and tankage) -6.5t (mass of engine)= 57.7t payload. Step 6 This is more lifting capacity than we need, so we use 1 engine. Step 7 So now that we have our number of engines and required wet to dry ratio, we can calculate the tank mass for our actual stage. (1.883-1)(1*6.5+50) _________________= Mt (9-1.883) .883(56.5)/7.117 = 7.01t Our empty tanks weigh 7.01t. Our fuel weighs 8 times as much; 56.1t. Our engine weighs 6.5t. Our payload weighs 50t All together, our launch vehicle with payload is 119.6t Plugging this into the rocket equation as a quick sanity check yields 1,801 m/sec DV Step 8 Our engine already carries 32t of LF&O, so we need to add another 24t. That's another x200-32 and X-200-16. Engine: $17,000 X200-32: $3,000 X200-16: $1,550 Stage cost: $21,550